Update JSON-RPC documentation

[CriesofCarrots]

Problem

JSON-RPC documentation has not remained current with implementation. A more programmatic solution is being explored (#7866), but in the meantime, the current solution ought to reflect reality.

Summary of Changes

• Update JSON-RPC documentation
• Also sneak in an improvement to getBlockCommitment clunkiness, discovered when updating docs:

Before:
"result":{"commitment":{"commitment":[0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,10,32]},"totalStake": 42}"
result":{"commitment":null,"totalStake": 42}

After:
"result":{"commitment":[0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,10,32],"totalStake": 42}
"result":{"commitment":null,"totalStake": 42}

Fixes #7808

[mvines]

awesome